You'll get better advice if you ask in the General Discussion or Strategy & Tips boards... but your people are tired of being packed in like sardines without anything to do. I bet they'd love to go see a movie, or play some bingo at church or something. Or, if all else fails, put a bunch of them in chains and work 'em to death: THAT'LL teach 'em to complain!
(translation: Your city has unhappy people due to a large population. There are various ways to increase the happiness rate, or you can switch to the Slavery civic to and "pop rush" things to reduce the population directly)